1. Field of the Invention
The present invention relates generally to local area networks, and more particularly to a network for effectively connecting a plurality of information nodes to a single cable in a local area network.
2. Prior Art
Local area networks, which permit digital data communications between personal computers, printers, work stations and similar data stations, are commonly used in offices, factories and similar locations to allow digital data communication, typically over coaxial cable or twisted pairs of transmission lines, between different computers and associated peripheral work stations at spaced locations within the office or factory facility. Local area networks thus, for example, permit users of personal computers to share applications programs, printers, plotters, and files, while independently employing the memories and computing power of the individual computers.
Several techniques, including the use of token-passing networks, are employed to control the transmission of digital data through the transmission lines between the computers to prevent two or more different streams of data from colliding, thereby to resolve the intended destinations of the data in the network.
One known local area token-passing network includes a plurality of multi-port transceiver-repeaters or hubs. In this system each node or computer station in the local area network is connected directly to one port of an active hub through a transceiver to a dedicated coaxial cable in a so-called modified tree structure. A network controller, which provides the necessary interface functions between the work station and computer or the like and the other portions of the network, is typically coupled to the cable by means of a transceiver. The transceiver processes the data to be transmitted from, or received by, the network controller for use by the network. Each of the transceivers in the network preferably has an output impedance that is substantially equal to the characteristic impedance of the cable, in accordance with conventional transmission line theory, which holds that signal reflection is avoided by terminating a cable with its characteristic impedance. However, if two of such transceivers are connected to one port of a hub, the cable would no longer be properly terminated in its characteristic impedance and signal transmission would then be deleteriously affected. The designer of the conventional local area network is thus typically required to supply one hub port for each node in the network, and a dedicated cable must be run from each node to a hub port. These requirements significantly increase both the cost and complexity of the local area network because of the relatively large amount of cables required and the number of active hub ports required for the network.
U.S. Pat. No. 4,775,864, assigned to the same assignee as the present invention, discloses a high-impedance transceiver for use in a local area network to connect a plurality of local area networks to a common cable which is terminated at its characteristic impedance, typically 93 ohms. The output impedance of each transceiver in the receiver node is high, in the order of 5,000 ohms. This allows a large number of work stations or the like to be connected through their respective network controllers to the cable without unduly loading down the cable. During the transmit mode, the impedance of the transceiver is low, but since only one personal computer is typically transmitting data on the network at any given time, this low impedance does not adversely affect network operation.
The high-impedance transceiver disclosed in the aforesaid patent has proven to be advantageous for this purpose, but it had certain disadvantages which limited its utility and acceptance by users of local area networks. One drawback of the high-impedance transceiver disclosed in the patent is that it may draw larger negative currents than are sometimes desired. Another drawback in this prior high-impedance transceiver is that if one personal computer or work station is turned off, such as when its user is ill or is on vacation, the off or powered-down computer can present a low impedance to the cable, thereby losing the advantage and defeating the purpose of the high-impedance transceiver.
It is an object of the present invention to provide an improved high-impedance transceiver for use in a multi-node local area network.
It is further object of the invention to provide a high-impedance transceiver of the type described which draws a lesser amount of negative current than in currently known transceivers of this type.
It is another object of the invention to provide a local area network of the type described in which the impedance of the transceiver remains high even when the computer or work station to which it is connected is powered down.